standards

title: standards(7)

summary: C and UNIX Standards

text:
# standards(7)

## NAME
standards - C and UNIX Standards

## DESCRIPTION
The STANDARDS section that appears in many manual pages identifies various standards to which the documented interface conforms. The following list briefly describes these standards.

V7

Version 7 (also known as Seventh Edition) UNIX, released by AT&T/Bell Labs in 1979. After this point, UNIX systems diverged into two main dialects: BSD and System V.

4.2BSD

This is an implementation standard defined by the 4.2 release of the Berkeley Software Distribution, released by the University of California at Berkeley. This was the first Berkeley release that contained a TCP/IP stack and the sockets API. 4.2BSD was released in 1983.

Earlier major BSD releases included 3BSD (1980), 4BSD (1980), and 4.1BSD (1981).

4.3BSD

The successor to 4.2BSD, released in 1986.

4.4BSD

The successor to 4.3BSD, released in 1993. This was the last major Berkeley release.

System V

This is an implementation standard defined by AT&T's milestone 1983 release of its commercial System V (five) release. The previous major AT&T release was System III, released in 1981.

System V release 2 (SVr2)

This was the next System V release, made in 1985. The SVr2 was formally described in the System V Interface Definition version 1 (SVID 1) published in 1985.

System V release 3 (SVr3)

This was the successor to SVr2, released in 1986. This release was formally described in the System V Interface Definition version 2 (SVID 2).

System V release 4 (SVr4)

This was the successor to SVr3, released in 1989. This version of System V is described in the "Programmer's Reference Manual: Operating System API (Intel processors)" (Prentice-Hall 1992, ISBN 0-13-951294-2) This release was formally described in the System V Interface Definition version 3 (SVID 3), and is considered the definitive System V release.

SVID 4

System V Interface Definition version 4, issued in 1995. Available online at http://www.sco.com/developers/devspecs/.

C89

This was the first C language standard, ratified by ANSI (American National Standards Institute) in 1989 (X3.159-1989). Sometimes this is known as ANSI C, but since C99 is also an ANSI standard, this term is ambiguous. This standard was also ratified by ISO (International Standards Organization) in 1990 (ISO/IEC 9899:1990), and is thus occasionally referred to as ISO C90.

C99

This revision of the C language standard was ratified by ISO in 1999 (ISO/IEC 9899:1999). Available online at http://www.open-std.org/jtc1/sc22/wg14/www/standards.

C11

This revision of the C language standard was ratified by ISO in 2011 (ISO/IEC 9899:2011).

LFS

The Large File Summit specification, completed in 1996. This specification defined mechanisms that allowed 32-bit systems to support the use of large files (i.e., 64-bit file offsets). See https://www.opengroup.org/platform/lfs.html.

POSIX.1-1988

This was the first POSIX standard, ratified by IEEE as IEEE Std 1003.1-1988, and subsequently adopted (with minor revisions) as an ISO standard in 1990. The term "POSIX" was coined by Richard Stallman.

POSIX.1-1990

"Portable Operating System Interface for Computing Environments". IEEE 1003.1-1990 part 1, ratified by ISO in 1990 (ISO/IEC 9945-1:1990).

POSIX.2

IEEE Std 1003.2-1992, describing commands and utilities, ratified by ISO in 1993 (ISO/IEC 9945-2:1993).

POSIX.1b (formerly known as POSIX.4)

IEEE Std 1003.1b-1993, describing real-time facilities for portable operating systems, ratified by ISO in 1996 (ISO/IEC 9945-1:1996).

POSIX.1c (formerly known as POSIX.4a)

IEEE Std 1003.1c-1995, which describes the POSIX threads interfaces.

POSIX.1d

IEEE Std 1003.1d-1999, which describes additional real-time extensions.

POSIX.1g

IEEE Std 1003.1g-2000, which describes networking APIs (including sockets).

POSIX.1j

IEEE Std 1003.1j-2000, which describes advanced real-time extensions.

POSIX.1-1996

A 1996 revision of POSIX.1 which incorporated POSIX.1b and POSIX.1c.

XPG3

Released in 1989, this was the first release of the X/Open Portability Guide to be based on a POSIX standard (POSIX.1-1988). This multivolume guide was developed by the X/Open Group, a multivendor consortium.

XPG4

A revision of the X/Open Portability Guide, released in 1992. This revision incorporated POSIX.2.

XPG4v2

A 1994 revision of XPG4. This is also referred to as Spec 1170, where 1170 referred to the number of interfaces defined by this standard.

SUS (SUSv1)

Single UNIX Specification. This was a repackaging of XPG4v2 and other X/Open standards (X/Open Curses Issue 4 version 2, X/Open Networking Service (XNS) Issue 4). Systems conforming to this standard can be branded UNIX 95.

SUSv2

Single UNIX Specification version 2. Sometimes also referred to (incorrectly) as XPG5. This standard appeared in 1997. Systems conforming to this standard can be branded UNIX 98. See also http://www.unix.org/version2/.)

POSIX.1-2001, SUSv3

This was a 2001 revision and consolidation of the POSIX.1, POSIX.2, and SUS standards into a single document, conducted under the auspices of the Austin Group http://www.opengroup.org/austin/. The standard is available online at http://www.unix.org/version3/.

The standard defines two levels of conformance: POSIX conformance, which is a baseline set of interfaces required of a conforming system; and XSI Conformance, which additionally mandates a set of interfaces (the "XSI extension") which are only optional for POSIX conformance. XSI-conformant systems can be branded UNIX 03.

The POSIX.1-2001 document is broken into four parts:

XBD: Definitions, terms, and concepts, header file specifications.

XSH: Specifications of functions (i.e., system calls and library functions in actual implementations).

XCU: Specifications of commands and utilities (i.e., the area formerly described by POSIX.2).

XRAT: Informative text on the other parts of the standard.

POSIX.1-2001 is aligned with C99, so that all of the library functions standardized in C99 are also standardized in POSIX.1-2001.

The Single UNIX Specification version 3 (SUSv3) comprises the Base Specifications containing XBD, XSH, XCU, and XRAT as above, plus X/Open Curses Issue 4 version 2 as an extra volume that is not in POSIX.1-2001.

Two Technical Corrigenda (minor fixes and improvements) of the original 2001 standard have occurred: TC1 in 2003 and TC2 in 2004.

POSIX.1-2008, SUSv4

Work on the next revision of POSIX.1/SUS was completed and ratified in 2008. The standard is available online at http://www.unix.org/version4/.

The changes in this revision are not as large as those that occurred for POSIX.1-2001/SUSv3, but a number of new interfaces are added and various details of existing specifications are modified. Many of the interfaces that were optional in POSIX.1-2001 become mandatory in the 2008 revision of the standard. A few interfaces that are present in POSIX.1-2001 are marked as obsolete in POSIX.1-2008, or removed from the standard altogether.

The revised standard is structured in the same way as its predecessor. The Single UNIX Specification version 4 (SUSv4) comprises the Base Specifications containing XBD, XSH, XCU, and XRAT, plus X/Open Curses Issue 7 as an extra volume that is not in POSIX.1-2008.

Again there are two levels of conformance: the baseline POSIX Conformance, and XSI Conformance, which mandates an additional set of interfaces beyond those in the base specification.

In general, where the STANDARDS section of a manual page lists POSIX.1-2001, it can be assumed that the interface also conforms to POSIX.1-2008, unless otherwise noted.

Technical Corrigendum 1 (minor fixes and improvements) of this standard was released in 2013.

Technical Corrigendum 2 of this standard was released in 2016.

Further information can be found on the Austin Group web site, http://www.opengroup.org/austin/.

SUSv4 2016 edition

This is equivalent to POSIX.1-2008, with the addition of Technical Corrigenda 1 and 2 and the XCurses specification.

POSIX.1-2017

This revision of POSIX is technically identical to POSIX.1-2008 with Technical Corrigenda 1 and 2 applied.

SUSv4 2018 edition

This is equivalent to POSIX.1-2017, with the addition of the XCurses specification.

The interfaces documented in POSIX.1/SUS are available as manual pages under sections 0p (header files), 1p (commands), and 3p (functions); thus one can write "man 3p open".

## SEE ALSO
getconf(1), confstr(3), pathconf(3), sysconf(3), attributes(7), feature_test_macros(7), libc(7), posixoptions(7), system_data_types(7)


package: man-pages

title: string_copying(7)

summary: copying strings and character sequences

text:
# string_copying(7)

## NAME
stpcpy, strcpy, strcat, stpecpy, strlcpy, strlcat, stpncpy, strncpy, zustr2ustp, zustr2stp, strncat, ustpcpy, ustr2stp - copying strings and character sequences

## SYNOPSIS

### Strings

```
// Chain-copy a string.
char *stpcpy(char *restrict dst, const char *restrict src);
```

```
// Copy/catenate a string.
char *strcpy(char *restrict dst, const char *restrict src);
char *strcat(char *restrict dst, const char *restrict src);
```

```
// Chain-copy a string with truncation.
char *stpecpy(char *dst, char end[0], const char *restrict src);
```

```
// Copy/catenate a string with truncation.
size_t strlcpy(char dst[restrict .sz], const char *restrict src,
               size_t sz);
size_t strlcat(char dst[restrict .sz], const char *restrict src,
               size_t sz);
```

### Null-padded character sequences

```
// Zero a fixed-width buffer, and
// copy a string into a character sequence with truncation.
char *stpncpy(char dst[restrict .sz], const char *restrict src,
               size_t sz);
```

```
// Zero a fixed-width buffer, and
// copy a string into a character sequence with truncation.
char *strncpy(char dst[restrict .sz], const char *restrict src,
               size_t sz);
```

```
// Chain-copy a null-padded character sequence into a character sequence.
char *zustr2ustp(char *restrict dst, const char src[restrict .sz],
               size_t sz);
```

```
// Chain-copy a null-padded character sequence into a string.
char *zustr2stp(char *restrict dst, const char src[restrict .sz],
               size_t sz);
```

```
// Catenate a null-padded character sequence into a string.
char *strncat(char *restrict dst, const char src[restrict .sz],
               size_t sz);
```

### Measured character sequences

```
// Chain-copy a measured character sequence.
char *ustpcpy(char *restrict dst, const char src[restrict .len],
               size_t len);
```

```
// Chain-copy a measured character sequence into a string.
char *ustr2stp(char *restrict dst, const char src[restrict .len],
               size_t len);
```

## DESCRIPTION

### Terms (and abbreviations)
string (str)

is a sequence of zero or more non-null characters followed by a null byte.

character sequence

is a sequence of zero or more non-null characters. A program should never use a character sequence where a string is required. However, with appropriate care, a string can be used in the place of a character sequence.

null-padded character sequence (zustr)

Character sequences can be contained in fixed-width buffers, which contain padding null bytes after the character sequence, to fill the rest of the buffer without affecting the character sequence; however, those padding null bytes are not part of the character sequence.

measured character sequence (ustr)

Character sequence delimited by its length. It may be a slice of a larger character sequence, or even of a string.

length (len)

is the number of non-null characters in a string or character sequence. It is the return value of strlen(str) and of strnlen(ustr, sz).

size (sz)

refers to the entire buffer where the string or character sequence is contained.

end

is the name of a pointer to one past the last element of a buffer. It is equivalent to &str[sz]. It is used as a sentinel value, to be able to truncate strings or character sequences instead of overrunning the containing buffer.

copy

This term is used when the writing starts at the first element pointed to by dst.

catenate

This term is used when a function first finds the terminating null byte in dst, and then starts writing at that position.

chain

This term is used when it's the programmer who provides a pointer to the terminating null byte in the string dst (or one after the last character in a character sequence), and the function starts writing at that location. The function returns a pointer to the new location of the terminating null byte (or one after the last character in a character sequence) after the call, so that the programmer can use it to chain such calls.

### Copy, catenate, and chain-copy
Originally, there was a distinction between functions that copy and those that catenate. However, newer functions that copy while allowing chaining cover both use cases with a single API. They are also algorithmically faster, since they don't need to search for the terminating null byte of the existing string. However, functions that catenate have a much simpler use, so if performance is not important, it can make sense to use them for improving readability.

The pointer returned by functions that allow chaining is a byproduct of the copy operation, so it has no performance costs. Functions that return such a pointer, and thus can be chained, have names of the form *stp*(), since it's common to name the pointer just p.

Chain-copying functions that truncate should accept a pointer to the end of the destination buffer, and have names of the form *stpe*(). This allows not having to recalculate the remaining size after each call.

### Truncate or not?
The first thing to note is that programmers should be careful with buffers, so they always have the correct size, and truncation is not necessary.

In most cases, truncation is not desired, and it is simpler to just do the copy. Simpler code is safer code. Programming against programming mistakes by adding more code just adds more points where mistakes can be made.

Nowadays, compilers can detect most programmer errors with features like compiler warnings, static analyzers, and _FORTIFY_SOURCE (see ftm(7)). Keeping the code simple helps these overflow-detection features be more precise.

When validating user input, however, it makes sense to truncate. Remember to check the return value of such function calls.

Functions that truncate:

•

stpecpy(3) is the most efficient string copy function that performs truncation. It only requires to check for truncation once after all chained calls.

•

strlcpy(3bsd) and strlcat(3bsd) are similar, but less efficient when chained.

•

stpncpy(3) and strncpy(3) also truncate, but they don't write strings, but rather null-padded character sequences.

### Null-padded character sequences
For historic reasons, some standard APIs, such as utmpx(5), use null-padded character sequences in fixed-width buffers. To interface with them, specialized functions need to be used.

To copy strings into them, use stpncpy(3).

To copy from an unterminated string within a fixed-width buffer into a string, ignoring any trailing null bytes in the source fixed-width buffer, you should use zustr2stp(3) or strncat(3).

To copy from an unterminated string within a fixed-width buffer into a character sequence, ignoring any trailing null bytes in the source fixed-width buffer, you should use zustr2ustp(3).

### Measured character sequences
The simplest character sequence copying function is mempcpy(3). It requires always knowing the length of your character sequences, for which structures can be used. It makes the code much faster, since you always know the length of your character sequences, and can do the minimal copies and length measurements. mempcpy(3) copies character sequences, so you need to explicitly set the terminating null byte if you need a string.

However, for keeping type safety, it's good to add a wrapper that uses char * instead of void *: ustpcpy(3).

In programs that make considerable use of strings or character sequences, and need the best performance, using overlapping character sequences can make a big difference. It allows holding subsequences of a larger character sequence, while not duplicating memory nor using time to do a copy.

However, this is delicate, since it requires using character sequences. C library APIs use strings, so programs that use character sequences will have to take care of differentiating strings from character sequences.

To copy a measured character sequence, use ustpcpy(3).

To copy a measured character sequence into a string, use ustr2stp(3).

Because these functions ask for the length, and a string is by nature composed of a character sequence of the same length plus a terminating null byte, a string is also accepted as input.

### String vs character sequence
Some functions only operate on strings. Those require that the input src is a string, and guarantee an output string (even when truncation occurs). Functions that catenate also require that dst holds a string before the call. List of functions:

•

stpcpy(3)

•

strcpy(3), strcat(3)

•

stpecpy(3)

•

strlcpy(3bsd), strlcat(3bsd)

Other functions require an input string, but create a character sequence as output. These functions have confusing names, and have a long history of misuse. List of functions:

•

stpncpy(3)

•

strncpy(3)

Other functions operate on an input character sequence, and create an output string. Functions that catenate also require that dst holds a string before the call. strncat(3) has an even more misleading name than the functions above. List of functions:

•

zustr2stp(3)

•

strncat(3)

•

ustr2stp(3)

Other functions operate on an input character sequence to create an output character sequence. List of functions:

•

ustpcpy(3)

•

zustr2stp(3)

### Functions
stpcpy(3)

This function copies the input string into a destination string. The programmer is responsible for allocating a buffer large enough. It returns a pointer suitable for chaining.

strcpy(3)

strcat(3)

These functions copy and catenate the input string into a destination string. The programmer is responsible for allocating a buffer large enough. The return value is useless.

stpcpy(3) is a faster alternative to these functions.

stpecpy(3)

This function copies the input string into a destination string. If the destination buffer, limited by a pointer to its end, isn't large enough to hold the copy, the resulting string is truncated (but it is guaranteed to be null-terminated). It returns a pointer suitable for chaining. Truncation needs to be detected only once after the last chained call.

This function is not provided by any library; see EXAMPLES for a reference implementation.

strlcpy(3bsd)

strlcat(3bsd)

These functions copy and catenate the input string into a destination string. If the destination buffer, limited by its size, isn't large enough to hold the copy, the resulting string is truncated (but it is guaranteed to be null-terminated). They return the length of the total string they tried to create.

stpecpy(3) is a simpler alternative to these functions.

stpncpy(3)

This function copies the input string into a destination null-padded character sequence in a fixed-width buffer. If the destination buffer, limited by its size, isn't large enough to hold the copy, the resulting character sequence is truncated. Since it creates a character sequence, it doesn't need to write a terminating null byte. It's impossible to distinguish truncation by the result of the call, from a character sequence that just fits the destination buffer; truncation should be detected by comparing the length of the input string with the size of the destination buffer.

strncpy(3)

This function is identical to stpncpy(3) except for the useless return value.

stpncpy(3) is a more useful alternative to this function.

zustr2ustp(3)

This function copies the input character sequence, contained in a null-padded fixed-width buffer, into a destination character sequence. The programmer is responsible for allocating a buffer large enough. It returns a pointer suitable for chaining.

A truncating version of this function doesn't exist, since the size of the original character sequence is always known, so it wouldn't be very useful.

This function is not provided by any library; see EXAMPLES for a reference implementation.

zustr2stp(3)

This function copies the input character sequence, contained in a null-padded fixed-width buffer, into a destination string. The programmer is responsible for allocating a buffer large enough. It returns a pointer suitable for chaining.

A truncating version of this function doesn't exist, since the size of the original character sequence is always known, so it wouldn't be very useful.

This function is not provided by any library; see EXAMPLES for a reference implementation.

strncat(3)

Do not confuse this function with strncpy(3); they are not related at all.

This function catenates the input character sequence, contained in a null-padded fixed-width buffer, into a destination string. The programmer is responsible for allocating a buffer large enough. The return value is useless.

zustr2stp(3) is a faster alternative to this function.

ustpcpy(3)

This function copies the input character sequence, limited by its length, into a destination character sequence. The programmer is responsible for allocating a buffer large enough. It returns a pointer suitable for chaining.

ustr2stp(3)

This function copies the input character sequence, limited by its length, into a destination string. The programmer is responsible for allocating a buffer large enough. It returns a pointer suitable for chaining.

## RETURN VALUE
The following functions return a pointer to the terminating null byte in the destination string.

•

stpcpy(3)

•

ustr2stp(3)

•

zustr2stp(3)

The following function returns a pointer to the terminating null byte in the destination string, except when truncation occurs; if truncation occurs, it returns a pointer to the end of the destination buffer.

•

stpecpy(3)

The following function returns a pointer to one after the last character in the destination character sequence; if truncation occurs, that pointer is equivalent to a pointer to the end of the destination buffer.

•

stpncpy(3)

The following functions return a pointer to one after the last character in the destination character sequence.

•

zustr2ustp(3)

•

ustpcpy(3)

The following functions return the length of the total string that they tried to create (as if truncation didn't occur).

•

strlcpy(3bsd), strlcat(3bsd)

The following functions return the dst pointer, which is useless.

•

strcpy(3), strcat(3)

•

strncpy(3)

•

strncat(3)

## NOTES
The Linux kernel has an internal function for copying strings, which is similar to stpecpy(3), except that it can't be chained:

strscpy(9)

This function copies the input string into a destination string. If the destination buffer, limited by its size, isn't large enough to hold the copy, the resulting string is truncated (but it is guaranteed to be null-terminated). It returns the length of the destination string, or -E2BIG on truncation.

stpecpy(3) is a simpler and faster alternative to this function.

## CAVEATS
Don't mix chain calls to truncating and non-truncating functions. It is conceptually wrong unless you know that the first part of a copy will always fit. Anyway, the performance difference will probably be negligible, so it will probably be more clear if you use consistent semantics: either truncating or non-truncating. Calling a non-truncating function after a truncating one is necessarily wrong.

## BUGS
All catenation functions share the same performance problem: Shlemiel the painter.

## EXAMPLES
The following are examples of correct use of each of these functions.

stpcpy(3)

```
p = buf;
p = stpcpy(p, "Hello ");
p = stpcpy(p, "world");
p = stpcpy(p, "!");
len = p - buf;
puts(buf);

```

strcpy(3)

strcat(3)

```
strcpy(buf, "Hello ");
strcat(buf, "world");
strcat(buf, "!");
len = strlen(buf);
puts(buf);

```

stpecpy(3)

```
end = buf + sizeof(buf);
p = buf;
p = stpecpy(p, end, "Hello ");
p = stpecpy(p, end, "world");
p = stpecpy(p, end, "!");
if (p == end) {

    p--;

    goto toolong;
}
len = p - buf;
puts(buf);

```

strlcpy(3bsd)

strlcat(3bsd)

```
if (strlcpy(buf, "Hello ", sizeof(buf)) >= sizeof(buf))

    goto toolong;
if (strlcat(buf, "world", sizeof(buf)) >= sizeof(buf))

    goto toolong;
len = strlcat(buf, "!", sizeof(buf));
if (len >= sizeof(buf))

    goto toolong;
puts(buf);

```

strscpy(9)

```
len = strscpy(buf, "Hello world!", sizeof(buf));
if (len == -E2BIG)

    goto toolong;
puts(buf);

```

stpncpy(3)

```
p = stpncpy(buf, "Hello world!", sizeof(buf));
if (sizeof(buf) < strlen("Hello world!"))

    goto toolong;
len = p - buf;
for (size_t i = 0; i < sizeof(buf); i++)

    putchar(buf[i]);

```

strncpy(3)

```
strncpy(buf, "Hello world!", sizeof(buf));
if (sizeof(buf) < strlen("Hello world!"))

    goto toolong;
len = strnlen(buf, sizeof(buf));
for (size_t i = 0; i < sizeof(buf); i++)

    putchar(buf[i]);

```

zustr2ustp(3)

```
p = buf;
p = zustr2ustp(p, "Hello ", 6);
p = zustr2ustp(p, "world", 42);  // Padding null bytes ignored.
p = zustr2ustp(p, "!", 1);
len = p - buf;
printf("%.*s\n", (int) len, buf);

```

zustr2stp(3)

```
p = buf;
p = zustr2stp(p, "Hello ", 6);
p = zustr2stp(p, "world", 42);  // Padding null bytes ignored.
p = zustr2stp(p, "!", 1);
len = p - buf;
puts(buf);

```

strncat(3)

```
buf[0] = '\0';  // There's no 'cpy' function to this 'cat'.
strncat(buf, "Hello ", 6);
strncat(buf, "world", 42);  // Padding null bytes ignored.
strncat(buf, "!", 1);
len = strlen(buf);
puts(buf);

```

ustpcpy(3)

```
p = buf;
p = ustpcpy(p, "Hello ", 6);
p = ustpcpy(p, "world", 5);
p = ustpcpy(p, "!", 1);
len = p - buf;
printf("%.*s\n", (int) len, buf);

```

ustr2stp(3)

```
p = buf;
p = ustr2stp(p, "Hello ", 6);
p = ustr2stp(p, "world", 5);
p = ustr2stp(p, "!", 1);
len = p - buf;
puts(buf);

```

### Implementations
Here are reference implementations for functions not provided by libc.

```
/* This code is in the public domain. */
char *
stpecpy(char *dst, char end[0], const char *restrict src)
{

    char *p;

    if (dst == NULL)

        return NULL;

    if (dst == end)

        return end;

    p = memccpy(dst, src, '\0', end - dst);

    if (p != NULL)

        return p - 1;

    /* truncation detected */

    end[-1] = '\0';

    return end;
}
char *
zustr2ustp(char *restrict dst, const char *restrict src, size_t sz)
{

    return ustpcpy(dst, src, strnlen(src, sz));
}
char *
zustr2stp(char *restrict dst, const char *restrict src, size_t sz)
{

    char  *p;

    p = zustr2ustp(dst, src, sz);

    *p = '\0';

    return p;
}
char *
ustpcpy(char *restrict dst, const char *restrict src, size_t len)
{

    return mempcpy(dst, src, len);
}
char *
ustr2stp(char *restrict dst, const char *restrict src, size_t len)
{

    char  *p;

    p = ustpcpy(dst, src, len);

    *p = '\0';

    return p;
}
```

## SEE ALSO
bzero(3), memcpy(3), memccpy(3), mempcpy(3), stpcpy(3), strlcpy(3bsd), strncat(3), stpncpy(3), string(3)

branch: sisyphus


suffixes

title: suffixes(7)

summary: list of file suffixes

text:
# suffixes(7)

## NAME
suffixes - list of file suffixes

## DESCRIPTION
It is customary to indicate the contents of a file with the file suffix, which (typically) consists of a period, followed by one or more letters. Many standard utilities, such as compilers, use this to recognize the type of file they are dealing with. The make(1) utility is driven by rules based on file suffix.

Following is a list of suffixes which are likely to be found on a Linux system.

Suffix

File type

,v

files for RCS (Revision Control System)

-

backup file

.C

C++ source code, equivalent to .cc

.F

Fortran source with cpp(1) directives

or file compressed using freeze

.S

assembler source with cpp(1) directives

.Y

file compressed using yabba

.Z

file compressed using compress(1)

.[0-9]+gf

TeX generic font files

.[0-9]+pk

TeX packed font files

.[1-9]

manual page for the corresponding section

.[1-9][a-z]

manual page for section plus subsection

.a

static object code library

.ad

X application default resource file

.ada

Ada source (may be body, spec, or combination)

.adb

Ada body source

.ads

Ada spec source

.afm

PostScript font metrics

.al

Perl autoload file

.am

automake(1) input file

.arc

arc(1) archive

.arj

arj(1) archive

.asc

PGP ASCII-armored data

.asm

(GNU) assembler source file

.au

Audio sound file

.aux

LaTeX auxiliary file

.avi

(msvideo) movie

.awk

AWK language program

.b

LILO boot loader image

.bak

backup file

.bash

bash(1) shell script

.bb

basic block list data produced by

gcc -ftest-coverage

.bbg

basic block graph data produced by

gcc -ftest-coverage

.bbl

BibTeX output

.bdf

X font file

.bib

TeX bibliographic database, BibTeX input

.bm

bitmap source

.bmp

bitmap

.bz2

file compressed using bzip2(1)

.c

C source

.cat

message catalog files

.cc

C++ source

.cf

configuration file

.cfg

configuration file

.cgi

WWW content generating script or program

.cls

LaTeX Class definition

.class

Java compiled byte-code

.conf

configuration file

.config

configuration file

.cpp

equivalent to .cc

.csh

csh(1) shell script

.cxx

equivalent to .cc

.dat

data file

.deb

Debian software package

.def

Modula-2 source for definition modules

.def

other definition files

.desc

initial part of mail message unpacked with

munpack(1)

.diff

file differences (diff(1) command output)

.dir

dbm data base directory file

.doc

documentation file

.dsc

Debian Source Control (source package)

.dtx

LaTeX package source file

.dvi

TeX's device independent output

.el

Emacs-Lisp source

.elc

compiled Emacs-Lisp source

.eps

encapsulated PostScript

.exp

Expect source code

.f

Fortran source

.f77

Fortran 77 source

.f90

Fortran 90 source

.fas

precompiled Common-Lisp

.fi

Fortran include files

.fig

FIG image file (used by xfig(1))

.fmt

TeX format file

.gif

Compuserve Graphics Image File format

.gmo

GNU format message catalog

.gsf

Ghostscript fonts

.gz

file compressed using gzip(1)

.h

C or C++ header files

.help

help file

.hf

equivalent to .help

.hlp

equivalent to .help

.htm

poor man's .html

.html

HTML document used with the World Wide Web

.hqx

7-bit encoded Macintosh file

.i

C source after preprocessing

.icon

bitmap source

.idx

reference or datum-index file for hypertext

or database system

.image

bitmap source

.in

configuration template, especially for GNU Autoconf

.info

files for the Emacs info browser

.info-[0-9]+

split info files

.ins

LaTeX package install file for docstrip

.itcl

itcl source code;

itcl ([incr Tcl]) is an OO extension of tcl

.java

a Java source file

.jpeg

Joint Photographic Experts Group format

.jpg

poor man's .jpeg

.js

JavaScript source code

.jsx

JSX (JavaScript XML-like extension) source code

.kmap

lyx(1) keymap

.l

equivalent to .lex or .lisp

.lex

lex(1) or flex(1) files

.lha

lharc archive

.lib

Common-Lisp library

.lisp

Lisp source

.ln

files for use with lint(1)

.log

log file, in particular produced by TeX

.lsm

Linux Software Map entry

.lsp

Common-Lisp source

.lzh

lharc archive

.m

Objective-C source code

.m4

m4(1) source

.mac

macro files for various programs

.man

manual page (usually source rather than formatted)

.map

map files for various programs

.me

Nroff source using the me macro package

.mf

Metafont (font generator for TeX) source

.mgp

MagicPoint file

.mm

sources for groff(1) in mm - format

.mo

Message catalog binary file

.mod

Modula-2 source for implementation modules

.mov

(quicktime) movie

.mp

Metapost source

.mp2

MPEG Layer 2 (audio) file

.mp3

MPEG Layer 3 (audio) file

.mpeg

movie file

.o

object file

.old

old or backup file

.orig

backup (original) version of a file, from patch(1)

.out

output file, often executable program (a.out)

.p

Pascal source

.pag

dbm data base data file

.patch

file differences for patch(1)

.pbm

portable bitmap format

.pcf

X11 font files

.pdf

Adobe Portable Data Format

(use Acrobat/acroread or xpdf)

.perl

Perl source (see .ph, .pl, and .pm)

.pfa

PostScript font definition files, ASCII format

.pfb

PostScript font definition files, binary format

.pgm

portable greymap format

.pgp

PGP binary data

.ph

Perl header file

.php

PHP program file

.php3

PHP3 program file

.pid

File to store daemon PID (e.g., crond.pid)

.pl

TeX property list file or Perl library file

.pm

Perl module

.png

Portable Network Graphics file

.po

Message catalog source

.pod

perldoc(1) file

.ppm

portable pixmap format

.pr

bitmap source

.ps

PostScript file

.py

Python source

.pyc

compiled python

.qt

quicktime movie

.r

RATFOR source (obsolete)

.rej

patches that patch(1) couldn't apply

.rpm

RPM software package

.rtf

Rich Text Format file

.rules

rules for something

.s

assembler source

.sa

stub libraries for a.out shared libraries

.sc

sc(1) spreadsheet commands

.scm

Scheme source code

.sed

sed source file

.sgml

SGML source file

.sh

sh(1) scripts

.shar

archive created by the shar(1) utility

.shtml

HTML using Server Side Includes

.so

Shared library or dynamically loadable object

.sql

SQL source

.sqml

SQML schema or query program

.sty

LaTeX style files

.sym

Modula-2 compiled definition modules

.tar

archive created by the tar(1) utility

.tar.Z

tar(1) archive compressed with compress(1)

.tar.bz2

tar(1) archive compressed with bzip2(1)

.tar.gz

tar(1) archive compressed with gzip(1)

.taz

tar(1) archive compressed with compress(1)

.tcl

tcl source code

.tex

TeX or LaTeX source

.texi

equivalent to .texinfo

.texinfo

Texinfo documentation source

.text

text file

.tfm

TeX font metric file

.tgz

tar archive compressed with gzip(1)

.tif

poor man's .tiff

.tiff

Tagged Image File Format

.tk

tcl/tk script

.tmp

temporary file

.tmpl

template files

.ts

TypeScript source code

.tsx

TypeScript with JSX source code (.ts + .jsx)

.txt

equivalent to .text

.uu

equivalent to .uue

.uue

binary file encoded with uuencode(1)

.vf

TeX virtual font file

.vpl

TeX virtual property list file

.w

Silvio Levi's CWEB

.wav

wave sound file

.web

Donald Knuth's WEB

.wml

Source file for Web Meta Language

.xbm

X11 bitmap source

.xcf

GIMP graphic

.xml

eXtended Markup Language file

.xpm

X11 pixmap source

.xs

Perl xsub file produced by h2xs

.xsl

XSL stylesheet

.y

yacc(1) or bison(1) (parser generator) files

.z

File compressed using pack(1) (or an old gzip(1))

.zip

zip(1) archive

.zoo

zoo(1) archive

~

Emacs or patch(1) backup file

rc

startup (`run control') file, e.g., .newsrc

## STANDARDS
General UNIX conventions.

## BUGS
This list is not exhaustive.

## SEE ALSO
file(1), make(1)


symlink

title: symlink(7)

summary: symbolic link handling

text:
# symlink(7)

## NAME
symlink - symbolic link handling

## DESCRIPTION
Symbolic links are files that act as pointers to other files. To understand their behavior, you must first understand how hard links work.

A hard link to a file is indistinguishable from the original file because it is a reference to the object underlying the original filename. (To be precise: each of the hard links to a file is a reference to the same inode number, where an inode number is an index into the inode table, which contains metadata about all files on a filesystem. See stat(2).) Changes to a file are independent of the name used to reference the file. Hard links may not refer to directories (to prevent the possibility of loops within the filesystem tree, which would confuse many programs) and may not refer to files on different filesystems (because inode numbers are not unique across filesystems).

A symbolic link is a special type of file whose contents are a string that is the pathname of another file, the file to which the link refers. (The contents of a symbolic link can be read using readlink(2).) In other words, a symbolic link is a pointer to another name, and not to an underlying object. For this reason, symbolic links may refer to directories and may cross filesystem boundaries.

There is no requirement that the pathname referred to by a symbolic link should exist. A symbolic link that refers to a pathname that does not exist is said to be a dangling link.

Because a symbolic link and its referenced object coexist in the filesystem name space, confusion can arise in distinguishing between the link itself and the referenced object. On historical systems, commands and system calls adopted their own link-following conventions in a somewhat ad-hoc fashion. Rules for a more uniform approach, as they are implemented on Linux and other systems, are outlined here. It is important that site-local applications also conform to these rules, so that the user interface can be as consistent as possible.

### Magic links
There is a special class of symbolic-link-like objects known as "magic links", which can be found in certain pseudofilesystems such as proc(5) (examples include /proc/pid/exe and /proc/pid/fd/*). Unlike normal symbolic links, magic links are not resolved through pathname-expansion, but instead act as direct references to the kernel's own representation of a file handle. As such, these magic links allow users to access files which cannot be referenced with normal paths (such as unlinked files still referenced by a running program ).

Because they can bypass ordinary mount_namespaces(7)-based restrictions, magic links have been used as attack vectors in various exploits.

### Symbolic link ownership, permissions, and timestamps
The owner and group of an existing symbolic link can be changed using lchown(2). The ownership of a symbolic link matters when the link is being removed or renamed in a directory that has the sticky bit set (see inode(7)), and when the fs.protected_symlinks sysctl is set (see proc(5)).

The last access and last modification timestamps of a symbolic link can be changed using utimensat(2) or lutimes(3).

On Linux, the permissions of an ordinary symbolic link are not used in any operations; the permissions are always 0777 (read, write, and execute for all user categories), and can't be changed.

However, magic links do not follow this rule. They can have a non-0777 mode, though this mode is not currently used in any permission checks.

### Obtaining a file descriptor that refers to a symbolic link
Using the combination of the O_PATH and O_NOFOLLOW flags to open(2) yields a file descriptor that can be passed as the dirfd argument in system calls such as fstatat(2), fchownat(2), fchmodat(2), linkat(2), and readlinkat(2), in order to operate on the symbolic link itself (rather than the file to which it refers).

By default (i.e., if the AT_SYMLINK_FOLLOW flag is not specified), if name_to_handle_at(2) is applied to a symbolic link, it yields a handle for the symbolic link (rather than the file to which it refers). One can then obtain a file descriptor for the symbolic link (rather than the file to which it refers) by specifying the O_PATH flag in a subsequent call to open_by_handle_at(2). Again, that file descriptor can be used in the aforementioned system calls to operate on the symbolic link itself.

### Handling of symbolic links by system calls and commands
Symbolic links are handled either by operating on the link itself, or by operating on the object referred to by the link. In the latter case, an application or system call is said to follow the link. Symbolic links may refer to other symbolic links, in which case the links are dereferenced until an object that is not a symbolic link is found, a symbolic link that refers to a file which does not exist is found, or a loop is detected. (Loop detection is done by placing an upper limit on the number of links that may be followed, and an error results if this limit is exceeded.)

There are three separate areas that need to be discussed. They are as follows:

•

Symbolic links used as filename arguments for system calls.

•

Symbolic links specified as command-line arguments to utilities that are not traversing a file tree.

•

Symbolic links encountered by utilities that are traversing a file tree (either specified on the command line or encountered as part of the file hierarchy walk).

Before describing the treatment of symbolic links by system calls and commands, we require some terminology. Given a pathname of the form a/b/c, the part preceding the final slash (i.e., a/b) is called the dirname component, and the part following the final slash (i.e., c) is called the basename component.

### Treatment of symbolic links in system calls
The first area is symbolic links used as filename arguments for system calls.

The treatment of symbolic links within a pathname passed to a system call is as follows:

(1)

Within the dirname component of a pathname, symbolic links are always followed in nearly every system call. (This is also true for commands.) The one exception is openat2(2), which provides flags that can be used to explicitly prevent following of symbolic links in the dirname component.

(2)

Except as noted below, all system calls follow symbolic links in the basename component of a pathname. For example, if there were a symbolic link slink which pointed to a file named afile, the system call open("slink" ...) would return a file descriptor referring to the file afile.

Various system calls do not follow links in the basename component of a pathname, and operate on the symbolic link itself. They are: lchown(2), lgetxattr(2), llistxattr(2), lremovexattr(2), lsetxattr(2), lstat(2), readlink(2), rename(2), rmdir(2), and unlink(2).

Certain other system calls optionally follow symbolic links in the basename component of a pathname. They are: faccessat(2), fchownat(2), fstatat(2), linkat(2), name_to_handle_at(2), open(2), openat(2), open_by_handle_at(2), and utimensat(2); see their manual pages for details. Because remove(3) is an alias for unlink(2), that library function also does not follow symbolic links. When rmdir(2) is applied to a symbolic link, it fails with the error ENOTDIR.

link(2) warrants special discussion. POSIX.1-2001 specifies that link(2) should dereference oldpath if it is a symbolic link. However, Linux does not do this. (By default, Solaris is the same, but the POSIX.1-2001 specified behavior can be obtained with suitable compiler options.) POSIX.1-2008 changed the specification to allow either behavior in an implementation.

### Commands not traversing a file tree
The second area is symbolic links, specified as command-line filename arguments, to commands which are not traversing a file tree.

Except as noted below, commands follow symbolic links named as command-line arguments. For example, if there were a symbolic link slink which pointed to a file named afile, the command cat slink would display the contents of the file afile.

It is important to realize that this rule includes commands which may optionally traverse file trees; for example, the command chown file is included in this rule, while the command chown -R file, which performs a tree traversal, is not. (The latter is described in the third area, below.)

If it is explicitly intended that the command operate on the symbolic link instead of following the symbolic link—for example, it is desired that chown slink change the ownership of the file that slink is, whether it is a symbolic link or not—then the -h option should be used. In the above example, chown root slink would change the ownership of the file referred to by slink, while chown -h root slink would change the ownership of slink itself.

There are some exceptions to this rule:

•

The mv(1) and rm(1) commands do not follow symbolic links named as arguments, but respectively attempt to rename and delete them. (Note, if the symbolic link references a file via a relative path, moving it to another directory may very well cause it to stop working, since the path may no longer be correct.)

•

The ls(1) command is also an exception to this rule. For compatibility with historic systems (when ls(1) is not doing a tree walk—that is, -R option is not specified), the ls(1) command follows symbolic links named as arguments if the -H or -L option is specified, or if the -F, -d, or -l options are not specified. (The ls(1) command is the only command where the -H and -L options affect its behavior even though it is not doing a walk of a file tree.)

•

The file(1) command is also an exception to this rule. The file(1) command does not follow symbolic links named as argument by default. The file(1) command does follow symbolic links named as argument if the -L option is specified.

### Commands traversing a file tree
The following commands either optionally or always traverse file trees: chgrp(1), chmod(1), chown(1), cp(1), du(1), find(1), ls(1), pax(1), rm(1), and tar(1).

It is important to realize that the following rules apply equally to symbolic links encountered during the file tree traversal and symbolic links listed as command-line arguments.

The first rule applies to symbolic links that reference files other than directories. Operations that apply to symbolic links are performed on the links themselves, but otherwise the links are ignored.

The command rm -r slink directory will remove slink, as well as any symbolic links encountered in the tree traversal of directory, because symbolic links may be removed. In no case will rm(1) affect the file referred to by slink.

The second rule applies to symbolic links that refer to directories. Symbolic links that refer to directories are never followed by default. This is often referred to as a "physical" walk, as opposed to a "logical" walk (where symbolic links that refer to directories are followed).

Certain conventions are (should be) followed as consistently as possible by commands that perform file tree walks:

•

A command can be made to follow any symbolic links named on the command line, regardless of the type of file they reference, by specifying the -H (for "half-logical") flag. This flag is intended to make the command-line name space look like the logical name space. (Note, for commands that do not always do file tree traversals, the -H flag will be ignored if the -R flag is not also specified.)

For example, the command chown -HR user slink will traverse the file hierarchy rooted in the file pointed to by slink. Note, the -H is not the same as the previously discussed -h flag. The -H flag causes symbolic links specified on the command line to be dereferenced for the purposes of both the action to be performed and the tree walk, and it is as if the user had specified the name of the file to which the symbolic link pointed.

•

A command can be made to follow any symbolic links named on the command line, as well as any symbolic links encountered during the traversal, regardless of the type of file they reference, by specifying the -L (for "logical") flag. This flag is intended to make the entire name space look like the logical name space. (Note, for commands that do not always do file tree traversals, the -L flag will be ignored if the -R flag is not also specified.)

For example, the command chown -LR user slink will change the owner of the file referred to by slink. If slink refers to a directory, chown will traverse the file hierarchy rooted in the directory that it references. In addition, if any symbolic links are encountered in any file tree that chown traverses, they will be treated in the same fashion as slink.

•

A command can be made to provide the default behavior by specifying the -P (for "physical") flag. This flag is intended to make the entire name space look like the physical name space.

For commands that do not by default do file tree traversals, the -H, -L, and -P flags are ignored if the -R flag is not also specified. In addition, you may specify the -H, -L, and -P options more than once; the last one specified determines the command's behavior. This is intended to permit you to alias commands to behave one way or the other, and then override that behavior on the command line.

The ls(1) and rm(1) commands have exceptions to these rules:

•

The rm(1) command operates on the symbolic link, and not the file it references, and therefore never follows a symbolic link. The rm(1) command does not support the -H, -L, or -P options.

•

To maintain compatibility with historic systems, the ls(1) command acts a little differently. If you do not specify the -F, -d, or -l options, ls(1) will follow symbolic links specified on the command line. If the -L flag is specified, ls(1) follows all symbolic links, regardless of their type, whether specified on the command line or encountered in the tree walk.

## SEE ALSO
chgrp(1), chmod(1), find(1), ln(1), ls(1), mv(1), namei(1), rm(1), lchown(2), link(2), lstat(2), readlink(2), rename(2), symlink(2), unlink(2), utimensat(2), lutimes(3), path_resolution(7)


title: system_data_types(7)

summary: overview of system data types

text:
# system_data_types(7)

## NAME
system_data_types - overview of system data types

## DESCRIPTION
sigevent

Include: <signal.h>. Alternatively, <aio.h>, <mqueue.h>, or <time.h>.

```
struct sigevent {

    int             sigev_notify; /* Notification type */

    int             sigev_signo;  /* Signal number */

    union sigval    sigev_value;  /* Signal value */

    void          (*sigev_notify_function)(union sigval);

                                  /* Notification function */

    pthread_attr_t *sigev_notify_attributes;

                                  /* Notification attributes */
};
```

For further details about this type, see sigevent(7).

Versions: <aio.h> and <time.h> define sigevent since POSIX.1-2008.

Conforming to: POSIX.1-2001 and later.

See also: timer_create(2), getaddrinfo_a(3), lio_listio(3), mq_notify(3)

See also the aiocb structure in this page.

siginfo_t

Include: <signal.h>. Alternatively, <sys/wait.h>.

```
typedef struct {

    int      si_signo;  /* Signal number */

    int      si_code;   /* Signal code */

    pid_t    si_pid;    /* Sending process ID */

    uid_t    si_uid;    /* Real user ID of sending process */

    void    *si_addr;   /* Address of faulting instruction */

    int      si_status; /* Exit value or signal */

    union sigval si_value;  /* Signal value */
} siginfo_t;
```

Information associated with a signal. For further details on this structure (including additional, Linux-specific fields), see sigaction(2).

Conforming to: POSIX.1-2001 and later.

See also: pidfd_send_signal(2), rt_sigqueueinfo(2), sigaction(2), sigwaitinfo(2), psiginfo(3)

sigset_t

Include: <signal.h>. Alternatively, <spawn.h>, or <sys/select.h>. This is a type that represents a set of signals. According to POSIX, this shall be an integer or structure type.

Conforming to: POSIX.1-2001 and later.

See also: epoll_pwait(2), ppoll(2), pselect(2), sigaction(2), signalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2), sigwaitinfo(2), signal(7)

sigval

Include: <signal.h>.

```
union sigval {

    int     sival_int; /* Integer value */

    void   *sival_ptr; /* Pointer value */
};
```

Data passed with a signal.

Conforming to: POSIX.1-2001 and later.

See also: pthread_sigqueue(3), sigqueue(3), sigevent(7)

See also the sigevent structure and the siginfo_t type in this page.

## NOTES
The structures described in this manual page shall contain, at least, the members shown in their definition, in no particular order.

Most of the integer types described in this page don't have a corresponding length modifier for the printf(3) and the scanf(3) families of functions. To print a value of an integer type that doesn't have a length modifier, it should be converted to intmax_t or uintmax_t by an explicit cast. To scan into a variable of an integer type that doesn't have a length modifier, an intermediate temporary variable of type intmax_t or uintmax_t should be used. When copying from the temporary variable to the destination variable, the value could overflow. If the type has upper and lower limits, the user should check that the value is within those limits, before actually copying the value. The example below shows how these conversions should be done.

### Conventions used in this page
In "Conforming to" we only concern ourselves with C99 and later and POSIX.1-2001 and later. Some types may be specified in earlier versions of one of these standards, but in the interests of simplicity we omit details from earlier standards.

In "Include", we first note the "primary" header(s) that define the type according to either the C or POSIX.1 standards. Under "Alternatively", we note additional headers that the standards specify shall define the type.

## EXAMPLES
The program shown below scans from a string and prints a value stored in a variable of an integer type that doesn't have a length modifier. The appropriate conversions from and to intmax_t, and the appropriate range checks, are used as explained in the notes section above.

```
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
int
main (void)
{

    static const char *const str = "500000 us in half a second";

    suseconds_t us;

    intmax_t    tmp;

    /* Scan the number from the string into the temporary variable. */

    sscanf(str, "%jd", &tmp);

    /* Check that the value is within the valid range of suseconds_t. */

    if (tmp < -1 || tmp > 1000000) {

        fprintf(stderr, "Scanned value outside valid range!\n");

        exit(EXIT_FAILURE);

    }

    /* Copy the value to the suseconds_t variable 'us'. */

    us = tmp;

    /* Even though suseconds_t can hold the value -1, this isn't

       a sensible number of microseconds. */

    if (us < 0) {

        fprintf(stderr, "Scanned value shouldn't be negative!\n");

        exit(EXIT_FAILURE);

    }

    /* Print the value. */

    printf("There are %jd microseconds in half a second.\n",

            (intmax_t) us);

    exit(EXIT_SUCCESS);
}
```

## SEE ALSO
feature_test_macros(7), standards(7)


sysvipc

title: sysvipc(7)

text:
# sysvipc(7)

## NAME
sysvipc - System V interprocess communication mechanisms

## DESCRIPTION
System V IPC is the name given to three interprocess communication mechanisms that are widely available on UNIX systems: message queues, semaphore, and shared memory.

### Message queues
System V message queues allow data to be exchanged in units called messages. Each message can have an associated priority. POSIX message queues provide an alternative API for achieving the same result; see mq_overview(7).

The System V message queue API consists of the following system calls:

msgget(2)

Create a new message queue or obtain the ID of an existing message queue. This call returns an identifier that is used in the remaining APIs.

msgsnd(2)

Add a message to a queue.

msgrcv(2)

Remove a message from a queue.

msgctl(2)

Perform various control operations on a queue, including deletion.

### Semaphore sets
System V semaphores allow processes to synchronize their actions. System V semaphores are allocated in groups called sets; each semaphore in a set is a counting semaphore. POSIX semaphores provide an alternative API for achieving the same result; see sem_overview(7).

The System V semaphore API consists of the following system calls:

semget(2)

Create a new set or obtain the ID of an existing set. This call returns an identifier that is used in the remaining APIs.

semop(2)

Perform operations on the semaphores in a set.

semctl(2)

Perform various control operations on a set, including deletion.

### Shared memory segments
System V shared memory allows processes to share a region a memory (a "segment"). POSIX shared memory is an alternative API for achieving the same result; see shm_overview(7).

The System V shared memory API consists of the following system calls:

shmget(2)

Create a new segment or obtain the ID of an existing segment. This call returns an identifier that is used in the remaining APIs.

shmat(2)

Attach an existing shared memory object into the calling process's address space.

shmdt(2)

Detach a segment from the calling process's address space.

shmctl(2)

Perform various control operations on a segment, including deletion.

### IPC namespaces
For a discussion of the interaction of System V IPC objects and IPC namespaces, see ipc_namespaces(7).

## SEE ALSO
ipcmk(1), ipcrm(1), ipcs(1), lsipc(1), ipc(2), msgctl(2), msgget(2), msgrcv(2), msgsnd(2), semctl(2), semget(2), semop(2), shmat(2), shmctl(2), shmdt(2), shmget(2), ftok(3), ipc_namespaces(7)


tcp

title: tcp(7)

summary: TCP protocol

text:
# tcp(7)

## NAME
tcp - TCP protocol

## SYNOPSIS

```
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
```

```
tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
```

## DESCRIPTION
This is an implementation of the TCP protocol defined in RFC 793, RFC 1122 and RFC 2001 with the NewReno and SACK extensions. It provides a reliable, stream-oriented, full-duplex connection between two sockets on top of ip(7), for both v4 and v6 versions. TCP guarantees that the data arrives in order and retransmits lost packets. It generates and checks a per-packet checksum to catch transmission errors. TCP does not preserve record boundaries.

A newly created TCP socket has no remote or local address and is not fully specified. To create an outgoing TCP connection use connect(2) to establish a connection to another TCP socket. To receive new incoming connections, first bind(2) the socket to a local address and port and then call listen(2) to put the socket into the listening state. After that a new socket for each incoming connection can be accepted using accept(2). A socket which has had accept(2) or connect(2) successfully called on it is fully specified and may transmit data. Data cannot be transmitted on listening or not yet connected sockets.

Linux supports RFC 1323 TCP high performance extensions. These include Protection Against Wrapped Sequence Numbers (PAWS), Window Scaling and Timestamps. Window scaling allows the use of large (> 64 kB) TCP windows in order to support links with high latency or bandwidth. To make use of them, the send and receive buffer sizes must be increased. They can be set globally with the /proc/sys/net/ipv4/tcp_wmem and /proc/sys/net/ipv4/tcp_rmem files, or on individual sockets by using the SO_SNDBUF and SO_RCVBUF socket options with the setsockopt(2) call.

The maximum sizes for socket buffers declared via the SO_SNDBUF and SO_RCVBUF mechanisms are limited by the values in the /proc/sys/net/core/rmem_max and /proc/sys/net/core/wmem_max files. Note that TCP actually allocates twice the size of the buffer requested in the setsockopt(2) call, and so a succeeding getsockopt(2) call will not return the same size of buffer as requested in the setsockopt(2) call. TCP uses the extra space for administrative purposes and internal kernel structures, and the /proc file values reflect the larger sizes compared to the actual TCP windows. On individual connections, the socket buffer size must be set prior to the listen(2) or connect(2) calls in order to have it take effect. See socket(7) for more information.

TCP supports urgent data. Urgent data is used to signal the receiver that some important message is part of the data stream and that it should be processed as soon as possible. To send urgent data specify the MSG_OOB option to send(2). When urgent data is received, the kernel sends a SIGURG signal to the process or process group that has been set as the socket "owner" using the SIOCSPGRP or FIOSETOWN ioctls (or the POSIX.1-specified fcntl(2) F_SETOWN operation). When the SO_OOBINLINE socket option is enabled, urgent data is put into the normal data stream (a program can test for its location using the SIOCATMARK ioctl described below), otherwise it can be received only when the MSG_OOB flag is set for recv(2) or recvmsg(2).

When out-of-band data is present, select(2) indicates the file descriptor as having an exceptional condition and poll (2) indicates a POLLPRI event.

Linux 2.4 introduced a number of changes for improved throughput and scaling, as well as enhanced functionality. Some of these features include support for zero-copy sendfile(2), Explicit Congestion Notification, new management of TIME_WAIT sockets, keep-alive socket options and support for Duplicate SACK extensions.

### Address formats
TCP is built on top of IP (see ip(7)). The address formats defined by ip(7) apply to TCP. TCP supports point-to-point communication only; broadcasting and multicasting are not supported.

### /proc interfaces
System-wide TCP parameter settings can be accessed by files in the directory /proc/sys/net/ipv4/. In addition, most IP /proc interfaces also apply to TCP; see ip(7). Variables described as Boolean take an integer value, with a nonzero value ("true") meaning that the corresponding option is enabled, and a zero value ("false") meaning that the option is disabled.

tcp_abc (Integer; default: 0; Linux 2.6.15 to Linux 3.8)

Control the Appropriate Byte Count (ABC), defined in RFC 3465. ABC is a way of increasing the congestion window (cwnd) more slowly in response to partial acknowledgements. Possible values are:

0

increase cwnd once per acknowledgement (no ABC)

1

increase cwnd once per acknowledgement of full sized segment

2

allow increase cwnd by two if acknowledgement is of two segments to compensate for delayed acknowledgements.

tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)

Enable resetting connections if the listening service is too slow and unable to keep up and accept them. It means that if overflow occurred due to a burst, the connection will recover. Enable this option only if you are really sure that the listening daemon cannot be tuned to accept connections faster. Enabling this option can harm the clients of your server.

tcp_adv_win_scale (integer; default: 2; since Linux 2.4)

Count buffering overhead as bytes/2^tcp_adv_win_scale, if tcp_adv_win_scale is greater than 0; or bytes-bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less than or equal to zero.

The socket receive buffer space is shared between the application and kernel. TCP maintains part of the buffer as the TCP window, this is the size of the receive window advertised to the other end. The rest of the space is used as the "application" buffer, used to isolate the network from scheduling and application latencies. The tcp_adv_win_scale default value of 2 implies that the space used for the application buffer is one fourth that of the total.

tcp_allowed_congestion_control (String; default: see text; since Linux 2.4.20)

Show/set the congestion control algorithm choices available to unprivileged processes (see the description of the TCP_CONGESTION socket option). The items in the list are separated by white space and terminated by a newline character. The list is a subset of those listed in tcp_available_congestion_control. The default value for this list is "reno" plus the default setting of tcp_congestion_control.

tcp_autocorking (Boolean; default: enabled; since Linux 3.14)

If this option is enabled, the kernel tries to coalesce small writes (from consecutive write(2) and sendmsg(2) calls) as much as possible, in order to decrease the total number of sent packets. Coalescing is done if at least one prior packet for the flow is waiting in Qdisc queues or device transmit queue. Applications can still use the TCP_CORK socket option to obtain optimal behavior when they know how/when to uncork their sockets.

tcp_available_congestion_control (String; read-only; since Linux 2.4.20)

Show a list of the congestion-control algorithms that are registered. The items in the list are separated by white space and terminated by a newline character. This list is a limiting set for the list in tcp_allowed_congestion_control. More congestion-control algorithms may be available as modules, but not loaded.

tcp_app_win (integer; default: 31; since Linux 2.4)

This variable defines how many bytes of the TCP window are reserved for buffering overhead.

A maximum of (window/2^tcp_app_win, mss) bytes in the window are reserved for the application buffer. A value of 0 implies that no amount is reserved.

tcp_base_mss (Integer; default: 512; since Linux 2.6.17)

The initial value of search_low to be used by the packetization layer Path MTU discovery (MTU probing). If MTU probing is enabled, this is the initial MSS used by the connection.

tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to Linux 2.6.13)

Enable BIC TCP congestion control algorithm. BIC-TCP is a sender-side-only change that ensures a linear RTT fairness under large windows while offering both scalability and bounded TCP-friendliness. The protocol combines two schemes called additive increase and binary search increase. When the congestion window is large, additive increase with a large increment ensures linear RTT fairness as well as good scalability. Under small congestion windows, binary search increase provides TCP friendliness.

tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to Linux 2.6.13)

Set the threshold window (in packets) where BIC TCP starts to adjust the congestion window. Below this threshold BIC TCP behaves the same as the default TCP Reno.

tcp_bic_fast_convergence (Boolean; default: enabled; Linux 2.4.27/2.6.6 to Linux 2.6.13)

Force BIC TCP to more quickly respond to changes in congestion window. Allows two flows sharing the same connection to converge more rapidly.

tcp_congestion_control (String; default: see text; since Linux 2.4.13)

Set the default congestion-control algorithm to be used for new connections. The algorithm "reno" is always available, but additional choices may be available depending on kernel configuration. The default value for this file is set as part of kernel configuration.

tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)

Lower limit, in bytes, of the size of socket reads that will be offloaded to a DMA copy engine, if one is present in the system and the kernel was configured with the CONFIG_NET_DMA option.

tcp_dsack (Boolean; default: enabled; since Linux 2.4)

Enable RFC 2883 TCP Duplicate SACK support.

tcp_fastopen (Bitmask; default: 0x1; since Linux 3.7)

Enables RFC 7413 Fast Open support. The flag is used as a bitmap with the following values:

0x1

Enables client side Fast Open support

0x2

Enables server side Fast Open support

0x4

Allows client side to transmit data in SYN without Fast Open option

0x200

Allows server side to accept SYN data without Fast Open option

0x400

Enables Fast Open on all listeners without TCP_FASTOPEN socket option

tcp_fastopen_key (since Linux 3.7)

Set server side RFC 7413 Fast Open key to generate Fast Open cookie when server side Fast Open support is enabled.

tcp_ecn (Integer; default: see below; since Linux 2.4)

Enable RFC 3168 Explicit Congestion Notification.

This file can have one of the following values:

0

Disable ECN. Neither initiate nor accept ECN. This was the default up to and including Linux 2.6.30.

1

Enable ECN when requested by incoming connections and also request ECN on outgoing connection attempts.

2

Enable ECN when requested by incoming connections, but do not request ECN on outgoing connections. This value is supported, and is the default, since Linux 2.6.31.

When enabled, connectivity to some destinations could be affected due to older, misbehaving middle boxes along the path, causing connections to be dropped. However, to facilitate and encourage deployment with option 1, and to work around such buggy equipment, the tcp_ecn_fallback option has been introduced.

tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)

Enable RFC 3168, Section 6.1.1.1. fallback. When enabled, outgoing ECN-setup SYNs that time out within the normal SYN retransmission timeout will be resent with CWR and ECE cleared.

tcp_fack (Boolean; default: enabled; since Linux 2.2)

Enable TCP Forward Acknowledgement support.

tcp_fin_timeout (integer; default: 60; since Linux 2.2)

This specifies how many seconds to wait for a final FIN packet before the socket is forcibly closed. This is strictly a violation of the TCP specification, but required to prevent denial-of-service attacks. In Linux 2.2, the default value was 180.

tcp_frto (integer; default: see below; since Linux 2.4.21/2.6)

Enable F-RTO, an enhanced recovery algorithm for TCP retransmission timeouts (RTOs). It is particularly beneficial in wireless environments where packet loss is typically due to random radio interference rather than intermediate router congestion. See RFC 4138 for more details.

This file can have one of the following values:

0

Disabled. This was the default up to and including Linux 2.6.23.

1

The basic version F-RTO algorithm is enabled.

2

Enable SACK-enhanced F-RTO if flow uses SACK. The basic version can be used also when SACK is in use though in that case scenario(s) exists where F-RTO interacts badly with the packet counting of the SACK-enabled TCP flow. This value is the default since Linux 2.6.24.

Before Linux 2.6.22, this parameter was a Boolean value, supporting just values 0 and 1 above.

tcp_frto_response (integer; default: 0; since Linux 2.6.22)

When F-RTO has detected that a TCP retransmission timeout was spurious (i.e., the timeout would have been avoided had TCP set a longer retransmission timeout), TCP has several options concerning what to do next. Possible values are:

0

Rate halving based; a smooth and conservative response, results in halved congestion window (cwnd) and slow-start threshold (ssthresh) after one RTT.

1

Very conservative response; not recommended because even though being valid, it interacts poorly with the rest of Linux TCP; halves cwnd and ssthresh immediately.

2

Aggressive response; undoes congestion-control measures that are now known to be unnecessary (ignoring the possibility of a lost retransmission that would require TCP to be more cautious); cwnd and ssthresh are restored to the values prior to timeout.

tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)

The number of seconds between TCP keep-alive probes.

tcp_keepalive_probes (integer; default: 9; since Linux 2.2)

The maximum number of TCP keep-alive probes to send before giving up and killing the connection if no response is obtained from the other end.

tcp_keepalive_time (integer; default: 7200; since Linux 2.2)

The number of seconds a connection needs to be idle before TCP begins sending out keep-alive probes. Keep-alives are sent only when the SO_KEEPALIVE socket option is enabled. The default value is 7200 seconds (2 hours). An idle connection is terminated after approximately an additional 11 minutes (9 probes an interval of 75 seconds apart) when keep-alive is enabled.

Note that underlying connection tracking mechanisms and application timeouts may be much shorter.

tcp_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6; obsolete since Linux 4.14)

If enabled, the TCP stack makes decisions that prefer lower latency as opposed to higher throughput. It this option is disabled, then higher throughput is preferred. An example of an application where this default should be changed would be a Beowulf compute cluster. Since Linux 4.14, this file still exists, but its value is ignored.

tcp_max_orphans (integer; default: see below; since Linux 2.4)

The maximum number of orphaned (not attached to any user file handle) TCP sockets allowed in the system. When this number is exceeded, the orphaned connection is reset and a warning is printed. This limit exists only to prevent simple denial-of-service attacks. Lowering this limit is not recommended. Network conditions might require you to increase the number of orphans allowed, but note that each orphan can eat up to ~64 kB of unswappable memory. The default initial value is set equal to the kernel parameter NR_FILE. This initial default is adjusted depending on the memory in the system.

tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)

The maximum number of queued connection requests which have still not received an acknowledgement from the connecting client. If this number is exceeded, the kernel will begin dropping requests. The default value of 256 is increased to 1024 when the memory present in the system is adequate or greater (>= 128 MB), and reduced to 128 for those systems with very low memory (<= 32 MB).

Before Linux 2.6.20, it was recommended that if this needed to be increased above 1024, the size of the SYNACK hash table (TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to keep

```
TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog

```

and the kernel should be recompiled. In Linux 2.6.20, the fixed sized TCP_SYNQ_HSIZE was removed in favor of dynamic sizing.

tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)

The maximum number of sockets in TIME_WAIT state allowed in the system. This limit exists only to prevent simple denial-of-service attacks. The default value of NR_FILE*2 is adjusted depending on the memory in the system. If this number is exceeded, the socket is closed and a warning is printed.

tcp_moderate_rcvbuf (Boolean; default: enabled; since Linux 2.4.17/2.6.7)

If enabled, TCP performs receive buffer auto-tuning, attempting to automatically size the buffer (no greater than tcp_rmem[2]) to match the size required by the path for full throughput.

tcp_mem (since Linux 2.4)

This is a vector of 3 integers: [low, pressure, high]. These bounds, measured in units of the system page size, are used by TCP to track its memory usage. The defaults are calculated at boot time from the amount of available memory. (TCP can only use low memory for this, which is limited to around 900 megabytes on 32-bit systems. 64-bit systems do not suffer this limitation.)

low

TCP doesn't regulate its memory allocation when the number of pages it has allocated globally is below this number.

pressure

When the amount of memory allocated by TCP exceeds this number of pages, TCP moderates its memory consumption. This memory pressure state is exited once the number of pages allocated falls below the low mark.

high

The maximum number of pages, globally, that TCP will allocate. This value overrides any other limits imposed by the kernel.

tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)

This parameter controls TCP Packetization-Layer Path MTU Discovery. The following values may be assigned to the file:

0

Disabled

1

Disabled by default, enabled when an ICMP black hole detected

2

Always enabled, use initial MSS of tcp_base_mss.

tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)

By default, TCP saves various connection metrics in the route cache when the connection closes, so that connections established in the near future can use these to set initial conditions. Usually, this increases overall performance, but it may sometimes cause performance degradation. If tcp_no_metrics_save is enabled, TCP will not cache metrics on closing connections.

tcp_orphan_retries (integer; default: 8; since Linux 2.4)

The maximum number of attempts made to probe the other end of a connection which has been closed by our end.

tcp_reordering (integer; default: 3; since Linux 2.4)

The maximum a packet can be reordered in a TCP packet stream without TCP assuming packet loss and going into slow start. It is not advisable to change this number. This is a packet reordering detection metric designed to minimize unnecessary back off and retransmits provoked by reordering of packets on a connection.

tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)

Try to send full-sized packets during retransmit.

tcp_retries1 (integer; default: 3; since Linux 2.2)

The number of times TCP will attempt to retransmit a packet on an established connection normally, without the extra effort of getting the network layers involved. Once we exceed this number of retransmits, we first have the network layer update the route if possible before each new retransmit. The default is the RFC specified minimum of 3.

tcp_retries2 (integer; default: 15; since Linux 2.2)

The maximum number of times a TCP packet is retransmitted in established state before giving up. The default value is 15, which corresponds to a duration of approximately between 13 to 30 minutes, depending on the retransmission timeout. The RFC 1122 specified minimum limit of 100 seconds is typically deemed too short.

tcp_rfc1337 (Boolean; default: disabled; since Linux 2.2)

Enable TCP behavior conformant with RFC 1337. When disabled, if a RST is received in TIME_WAIT state, we close the socket immediately without waiting for the end of the TIME_WAIT period.

tcp_rmem (since Linux 2.4)

This is a vector of 3 integers: [min, default, max]. These parameters are used by TCP to regulate receive buffer sizes. TCP dynamically adjusts the size of the receive buffer from the defaults listed below, in the range of these values, depending on memory available in the system.

min

minimum size of the receive buffer used by each TCP socket. The default value is the system page size. (On Linux 2.4, the default value is 4 kB, lowered to PAGE_SIZE bytes in low-memory systems.) This value is used to ensure that in memory pressure mode, allocations below this size will still succeed. This is not used to bound the size of the receive buffer declared using SO_RCVBUF on a socket.

default

the default size of the receive buffer for a TCP socket. This value overwrites the initial default buffer size from the generic global net.core.rmem_default defined for all protocols. The default value is 87380 bytes. (On Linux 2.4, this will be lowered to 43689 in low-memory systems.) If larger receive buffer sizes are desired, this value should be increased (to affect all sockets). To employ large TCP windows, the net.ipv4.tcp_window_scaling must be enabled (default).

max

the maximum size of the receive buffer used by each TCP socket. This value does not override the global net.core.rmem_max. This is not used to limit the size of the receive buffer declared using SO_RCVBUF on a socket. The default value is calculated using the formula

```
max(87380, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))

```

(On Linux 2.4, the default is 87380*2 bytes, lowered to 87380 in low-memory systems).

tcp_sack (Boolean; default: enabled; since Linux 2.2)

Enable RFC 2018 TCP Selective Acknowledgements.

tcp_slow_start_after_idle (Boolean; default: enabled; since Linux 2.6.18)

If enabled, provide RFC 2861 behavior and time out the congestion window after an idle period. An idle period is defined as the current RTO (retransmission timeout). If disabled, the congestion window will not be timed out after an idle period.

tcp_stdurg (Boolean; default: disabled; since Linux 2.2)

If this option is enabled, then use the RFC 1122 interpretation of the TCP urgent-pointer field. According to this interpretation, the urgent pointer points to the last byte of urgent data. If this option is disabled, then use the BSD-compatible interpretation of the urgent pointer: the urgent pointer points to the first byte after the urgent data. Enabling this option may lead to interoperability problems.

tcp_syn_retries (integer; default: 6; since Linux 2.2)

The maximum number of times initial SYNs for an active TCP connection attempt will be retransmitted. This value should not be higher than 255. The default value is 6, which corresponds to retrying for up to approximately 127 seconds. Before Linux 3.7, the default value was 5, which (in conjunction with calculation based on other kernel parameters) corresponded to approximately 180 seconds.

tcp_synack_retries (integer; default: 5; since Linux 2.2)

The maximum number of times a SYN/ACK segment for a passive TCP connection will be retransmitted. This number should not be higher than 255.

tcp_syncookies (integer; default: 1; since Linux 2.2)

Enable TCP syncookies. The kernel must be compiled with CONFIG_SYN_COOKIES. The syncookies feature attempts to protect a socket from a SYN flood attack. This should be used as a last resort, if at all. This is a violation of the TCP protocol, and conflicts with other areas of TCP such as TCP extensions. It can cause problems for clients and relays. It is not recommended as a tuning mechanism for heavily loaded servers to help with overloaded or misconfigured conditions. For recommended alternatives see tcp_max_syn_backlog, tcp_synack_retries, and tcp_abort_on_overflow. Set to one of the following values:

0

Disable TCP syncookies.

1

Send out syncookies when the syn backlog queue of a socket overflows.

2

(since Linux 3.12) Send out syncookies unconditionally. This can be useful for network testing.

tcp_timestamps (integer; default: 1; since Linux 2.2)

Set to one of the following values to enable or disable RFC 1323 TCP timestamps:

0

Disable timestamps.

1

Enable timestamps as defined in RFC1323 and use random offset for each connection rather than only using the current time.

2

As for the value 1, but without random offsets. Setting tcp_timestamps to this value is meaningful since Linux 4.10.

tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)

This parameter controls what percentage of the congestion window can be consumed by a single TCP Segmentation Offload (TSO) frame. The setting of this parameter is a tradeoff between burstiness and building larger TSO frames.

tcp_tw_recycle (Boolean; default: disabled; Linux 2.4 to Linux 4.11)

Enable fast recycling of TIME_WAIT sockets. Enabling this option is not recommended as the remote IP may not use monotonically increasing timestamps (devices behind NAT, devices with per-connection timestamp offsets). See RFC 1323 (PAWS) and RFC 6191.

tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)

Allow to reuse TIME_WAIT sockets for new connections when it is safe from protocol viewpoint. It should not be changed without advice/request of technical experts.

tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to Linux 2.6.13)

Enable TCP Vegas congestion avoidance algorithm. TCP Vegas is a sender-side-only change to TCP that anticipates the onset of congestion by estimating the bandwidth. TCP Vegas adjusts the sending rate by modifying the congestion window. TCP Vegas should provide less packet loss, but it is not as aggressive as TCP Reno.

tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to Linux 2.6.13)

Enable TCP Westwood+ congestion control algorithm. TCP Westwood+ is a sender-side-only modification of the TCP Reno protocol stack that optimizes the performance of TCP congestion control. It is based on end-to-end bandwidth estimation to set congestion window and slow start threshold after a congestion episode. Using this estimation, TCP Westwood+ adaptively sets a slow start threshold and a congestion window which takes into account the bandwidth used at the time congestion is experienced. TCP Westwood+ significantly increases fairness with respect to TCP Reno in wired networks and throughput over wireless links.

tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)

Enable RFC 1323 TCP window scaling. This feature allows the use of a large window (> 64 kB) on a TCP connection, should the other end support it. Normally, the 16 bit window length field in the TCP header limits the window size to less than 64 kB. If larger windows are desired, applications can increase the size of their socket buffers and the window scaling option will be employed. If tcp_window_scaling is disabled, TCP will not negotiate the use of window scaling with the other end during connection setup.

tcp_wmem (since Linux 2.4)

This is a vector of 3 integers: [min, default, max]. These parameters are used by TCP to regulate send buffer sizes. TCP dynamically adjusts the size of the send buffer from the default values listed below, in the range of these values, depending on memory available.

min

Minimum size of the send buffer used by each TCP socket. The default value is the system page size. (On Linux 2.4, the default value is 4 kB.) This value is used to ensure that in memory pressure mode, allocations below this size will still succeed. This is not used to bound the size of the send buffer declared using SO_SNDBUF on a socket.

default

The default size of the send buffer for a TCP socket. This value overwrites the initial default buffer size from the generic global /proc/sys/net/core/wmem_default defined for all protocols. The default value is 16 kB. If larger send buffer sizes are desired, this value should be increased (to affect all sockets). To employ large TCP windows, the /proc/sys/net/ipv4/tcp_window_scaling must be set to a nonzero value (default).

max

The maximum size of the send buffer used by each TCP socket. This value does not override the value in /proc/sys/net/core/wmem_max. This is not used to limit the size of the send buffer declared using SO_SNDBUF on a socket. The default value is calculated using the formula

```
max(65536, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))

```

(On Linux 2.4, the default value is 128 kB, lowered 64 kB depending on low-memory systems.)

tcp_workaround_signed_windows (Boolean; default: disabled; since Linux 2.6.26)

If enabled, assume that no receipt of a window-scaling option means that the remote TCP is broken and treats the window as a signed quantity. If disabled, assume that the remote TCP is not broken even if we do not receive a window scaling option from it.

### Socket options
To set or get a TCP socket option, call getsockopt(2) to read or setsockopt(2) to write the option with the option level argument set to IPPROTO_TCP. Unless otherwise noted, optval is a pointer to an int. In addition, most IPPROTO_IP socket options are valid on TCP sockets. For more information see ip(7).

Following is a list of TCP-specific socket options. For details of some other socket options that are also applicable for TCP sockets, see socket(7).

TCP_CONGESTION (since Linux 2.6.13)

The argument for this option is a string. This option allows the caller to set the TCP congestion control algorithm to be used, on a per-socket basis. Unprivileged processes are restricted to choosing one of the algorithms in tcp_allowed_congestion_control (described above). Privileged processes (CAP_NET_ADMIN) can choose from any of the available congestion-control algorithms (see the description of tcp_available_congestion_control above).

TCP_CORK (since Linux 2.2)

If set, don't send out partial frames. All queued partial frames are sent when the option is cleared again. This is useful for prepending headers before calling sendfile(2), or for throughput optimization. As currently implemented, there is a 200 millisecond ceiling on the time for which output is corked by TCP_CORK. If this ceiling is reached, then queued data is automatically transmitted. This option can be combined with TCP_NODELAY only since Linux 2.5.71. This option should not be used in code intended to be portable.

TCP_DEFER_ACCEPT (since Linux 2.4)

Allow a listener to be awakened only when data arrives on the socket. Takes an integer value (seconds), this can bound the maximum number of attempts TCP will make to complete the connection. This option should not be used in code intended to be portable.

TCP_INFO (since Linux 2.4)

Used to collect information about this socket. The kernel returns a struct tcp_info as defined in the file /usr/include/linux/tcp.h. This option should not be used in code intended to be portable.

TCP_KEEPCNT (since Linux 2.4)

The maximum number of keepalive probes TCP should send before dropping the connection. This option should not be used in code intended to be portable.

TCP_KEEPIDLE (since Linux 2.4)

The time (in seconds) the connection needs to remain idle before TCP starts sending keepalive probes, if the socket option SO_KEEPALIVE has been set on this socket. This option should not be used in code intended to be portable.

TCP_KEEPINTVL (since Linux 2.4)

The time (in seconds) between individual keepalive probes. This option should not be used in code intended to be portable.

TCP_LINGER2 (since Linux 2.4)

The lifetime of orphaned FIN_WAIT2 state sockets. This option can be used to override the system-wide setting in the file /proc/sys/net/ipv4/tcp_fin_timeout for this socket. This is not to be confused with the socket(7) level option SO_LINGER. This option should not be used in code intended to be portable.

TCP_MAXSEG

The maximum segment size for outgoing TCP packets. In Linux 2.2 and earlier, and in Linux 2.6.28 and later, if this option is set before connection establishment, it also changes the MSS value announced to the other end in the initial packet. Values greater than the (eventual) interface MTU have no effect. TCP will also impose its minimum and maximum bounds over the value provided.

TCP_NODELAY

If set, disable the Nagle algorithm. This means that segments are always sent as soon as possible, even if there is only a small amount of data. When not set, data is buffered until there is a sufficient amount to send out, thereby avoiding the frequent sending of small packets, which results in poor utilization of the network. This option is overridden by TCP_CORK; however, setting this option forces an explicit flush of pending output, even if TCP_CORK is currently set.

TCP_QUICKACK (since Linux 2.4.4)

Enable quickack mode if set or disable quickack mode if cleared. In quickack mode, acks are sent immediately, rather than delayed if needed in accordance to normal TCP operation. This flag is not permanent, it only enables a switch to or from quickack mode. Subsequent operation of the TCP protocol will once again enter/leave quickack mode depending on internal protocol processing and factors such as delayed ack timeouts occurring and data transfer. This option should not be used in code intended to be portable.

TCP_SYNCNT (since Linux 2.4)

Set the number of SYN retransmits that TCP should send before aborting the attempt to connect. It cannot exceed 255. This option should not be used in code intended to be portable.

TCP_USER_TIMEOUT (since Linux 2.6.37)

This option takes an unsigned int as an argument. When the value is greater than 0, it specifies the maximum amount of time in milliseconds that transmitted data may remain unacknowledged, or buffered data may remain untransmitted (due to zero window size) before TCP will forcibly close the corresponding connection and return ETIMEDOUT to the application. If the option value is specified as 0, TCP will use the system default.

Increasing user timeouts allows a TCP connection to survive extended periods without end-to-end connectivity. Decreasing user timeouts allows applications to "fail fast", if so desired. Otherwise, failure may take up to 20 minutes with the current system defaults in a normal WAN environment.

This option can be set during any state of a TCP connection, but is effective only during the synchronized states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, and LAST-ACK). Moreover, when used with the TCP keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will override keepalive to determine when to close a connection due to keepalive failure.

The option has no effect on when TCP retransmits a packet, nor when a keepalive probe is sent.

This option, like many others, will be inherited by the socket returned by accept(2), if it was set on the listening socket.

Further details on the user timeout feature can be found in RFC 793 and RFC 5482 ("TCP User Timeout Option").

TCP_WINDOW_CLAMP (since Linux 2.4)

Bound the size of the advertised window to this value. The kernel imposes a minimum size of SOCK_MIN_RCVBUF/2. This option should not be used in code intended to be portable.

TCP_FASTOPEN (since Linux 3.6)

This option enables Fast Open (RFC 7413) on the listener socket. The value specifies the maximum length of pending SYNs (similar to the backlog argument in listen(2)). Once enabled, the listener socket grants the TCP Fast Open cookie on incoming SYN with TCP Fast Open option.

More importantly it accepts the data in SYN with a valid Fast Open cookie and responds SYN-ACK acknowledging both the data and the SYN sequence. accept(2) returns a socket that is available for read and write when the handshake has not completed yet. Thus the data exchange can commence before the handshake completes. This option requires enabling the server-side support on sysctl net.ipv4.tcp_fastopen (see above). For TCP Fast Open client-side support, see send(2) MSG_FASTOPEN or TCP_FASTOPEN_CONNECT below.

TCP_FASTOPEN_CONNECT (since Linux 4.11)

This option enables an alternative way to perform Fast Open on the active side (client). When this option is enabled, connect(2) would behave differently depending on if a Fast Open cookie is available for the destination.

If a cookie is not available (i.e. first contact to the destination), connect(2) behaves as usual by sending a SYN immediately, except the SYN would include an empty Fast Open cookie option to solicit a cookie.

If a cookie is available, connect(2) would return 0 immediately but the SYN transmission is deferred. A subsequent write(2) or sendmsg(2) would trigger a SYN with data plus cookie in the Fast Open option. In other words, the actual connect operation is deferred until data is supplied.

Note: While this option is designed for convenience, enabling it does change the behaviors and certain system calls might set different errno values. With cookie present, write(2) or sendmsg(2) must be called right after connect(2) in order to send out SYN+data to complete 3WHS and establish connection. Calling read(2) right after connect(2) without write(2) will cause the blocking socket to be blocked forever.

The application should either set TCP_FASTOPEN_CONNECT socket option before write(2) or sendmsg(2), or call write(2) or sendmsg(2) with MSG_FASTOPEN flag directly, instead of both on the same connection.

Here is the typical call flow with this new option:

```
s = socket();
setsockopt(s, IPPROTO_TCP, TCP_FASTOPEN_CONNECT, 1, ...);
connect(s);
write(s); /* write() should always follow connect()

           * in order to trigger SYN to go out. */
read(s)/write(s);
/* ... */
close(s);

```

### Sockets API
TCP provides limited support for out-of-band data, in the form of (a single byte of) urgent data. In Linux this means if the other end sends newer out-of-band data the older urgent data is inserted as normal data into the stream (even when SO_OOBINLINE is not set). This differs from BSD-based stacks.

Linux uses the BSD compatible interpretation of the urgent pointer field by default. This violates RFC 1122, but is required for interoperability with other stacks. It can be changed via /proc/sys/net/ipv4/tcp_stdurg.

It is possible to peek at out-of-band data using the recv(2) MSG_PEEK flag.

Since Linux 2.4, Linux supports the use of MSG_TRUNC in the flags argument of recv(2) (and recvmsg(2)). This flag causes the received bytes of data to be discarded, rather than passed back in a caller-supplied buffer. Since Linux 2.4.4, MSG_TRUNC also has this effect when used in conjunction with MSG_OOB to receive out-of-band data.

### Ioctls
The following ioctl(2) calls return information in value. The correct syntax is:

```
int value;
error = ioctl(tcp_socket, ioctl_type, &value);
```

ioctl_type is one of the following:

SIOCINQ

Returns the amount of queued unread data in the receive buffer. The socket must not be in LISTEN state, otherwise an error (EINVAL) is returned. SIOCINQ is defined in <linux/sockios.h>. Alternatively, you can use the synonymous FIONREAD, defined in <sys/ioctl.h>.

SIOCATMARK

Returns true (i.e., value is nonzero) if the inbound data stream is at the urgent mark.

If the SO_OOBINLINE socket option is set, and SIOCATMARK returns true, then the next read from the socket will return the urgent data. If the SO_OOBINLINE socket option is not set, and SIOCATMARK returns true, then the next read from the socket will return the bytes following the urgent data (to actually read the urgent data requires the recv(MSG_OOB) flag).

Note that a read never reads across the urgent mark. If an application is informed of the presence of urgent data via select(2) (using the exceptfds argument) or through delivery of a SIGURG signal, then it can advance up to the mark using a loop which repeatedly tests SIOCATMARK and performs a read (requesting any number of bytes) as long as SIOCATMARK returns false.

SIOCOUTQ

Returns the amount of unsent data in the socket send queue. The socket must not be in LISTEN state, otherwise an error (EINVAL) is returned. SIOCOUTQ is defined in <linux/sockios.h>. Alternatively, you can use the synonymous TIOCOUTQ, defined in <sys/ioctl.h>.

### Error handling
When a network error occurs, TCP tries to resend the packet. If it doesn't succeed after some time, either ETIMEDOUT or the last received error on this connection is reported.

Some applications require a quicker error notification. This can be enabled with the IPPROTO_IP level IP_RECVERR socket option. When this option is enabled, all incoming errors are immediately passed to the user program. Use this option with care — it makes TCP less tolerant to routing changes and other normal network conditions.

## ERRORS
EAFNOTSUPPORT

Passed socket address type in sin_family was not AF_INET.

EPIPE

The other end closed the socket unexpectedly or a read is executed on a shut down socket.

ETIMEDOUT

The other end didn't acknowledge retransmitted data after some time.

Any errors defined for ip(7) or the generic socket layer may also be returned for TCP.

## VERSIONS
Support for Explicit Congestion Notification, zero-copy sendfile(2), reordering support and some SACK extensions (DSACK) were introduced in Linux 2.4. Support for forward acknowledgement (FACK), TIME_WAIT recycling, and per-connection keepalive socket options were introduced in Linux 2.3.

## BUGS
Not all errors are documented.

IPv6 is not described.

## SEE ALSO
accept(2), bind(2), connect(2), getsockopt(2), listen(2), recvmsg(2), sendfile(2), sendmsg(2), socket(2), ip(7), socket(7)

The kernel source file Documentation/networking/ip-sysctl.txt.

RFC 793 for the TCP specification.

RFC 1122 for the TCP requirements and a description of the Nagle algorithm.

RFC 1323 for TCP timestamp and window scaling options.

RFC 1337 for a description of TIME_WAIT assassination hazards.

RFC 3168 for a description of Explicit Congestion Notification.

RFC 2581 for TCP congestion control algorithms.

RFC 2018 and RFC 2883 for SACK and extensions to SACK.